Engage the x drive: Ten ways to traverse deep space

In 1961, Yuri Gagarin became the first human being to reach outer space. Eight years later, Neil Armstrong and Buzz Aldrin made it to the surface of the moon. And that is as far as any of us has ventured.

Apart from the mundane problems of budgets and political will, the major roadblock is that our dominant space-flight technology – chemically fuelled rockets – just isn't up to the distances involved. We can send robot probes to the outer planets, but they take years to get there.

And as for visiting other stars, forget it. As an example of why, the Apollo 10 moon probe is currently listed as the fastest manned vehicle in history, having reached a maximum speed of 39,895 kilometres per hour. At this speed, it would take 120,000 years to cover the 4 light years to Alpha Centauri, the nearest star system.

The technologies range widely in their plausibility. Some, we could more or less build tomorrow if we wanted to, while others may well be fundamentally impossible.

Ion thruster

Conventional rockets work by shooting gases out of their rear exhausts at high speeds, thus generating thrust. Ion thrusters use the same principle, but instead of blasting out hot gases, they shoot out a beam of electrically charged particles, or ions.

They provide quite a weak thrust, but crucially they use far less fuel than a rocket to get the same amount of thrust. Providing they can be made to keep working steadily for a long time, they could eventually accelerate a craft to high speeds.

A particularly promising variant is the variable specific impulse magnetoplasma rocket (VASIMR). This works on a slightly different principle to other ion thrusters, which accelerate the ions using a strong electric field. Instead, VASIMR uses a radio-frequency generator, rather like the transmitters used to broadcast radio shows, to heat ions to 1 million °C.

It does this by taking advantage of the fact that in a strong magnetic field, like those produced by the superconducting magnets in the engine, ions spin at a fixed frequency. The radio-frequency generator is then tuned to that frequency, injecting extra energy into the ions and massively increasing the thrust.

Nuclear pulse propulsion

If some of the ideas here strike you as a little unlikely, this one will seem downright reckless. The notion here is to power your spacecraft by periodically throwing a nuclear bomb out of the back and setting it off.

The design DARPA came up with was huge even by today's standards, and was built to be a giant shock absorber, with heavy radiation shielding to protect the passengers.

It seemed workable, but there were concerns about fallout if it was launched in the atmosphere as planned. The project was eventually dropped in the 1960s when the first nuclear test bans came into force.

Despite these worries, the Orion design remains one that could be built using existing technology, and some researchers are still coming up with new approaches to nuclear pulse propulsion. Theoretically, a nuclear-bomb-powered ship could reach up to 10 per cent of the speed of light, allowing a journey to the nearest star in about 40 years.

Plausibility: perfectly possible, if a tad hazardous

Fusion rocket

Nuclear pulse propulsion is far from the only space-flight technology that depends on nuclear power.

For instance, nuclear rockets could use the heat from an onboard fission reactor to expel gases, providing thrust. But in terms of power, these pale in comparison to fusion rockets.

Nuclear fusion, in which the nuclei of atoms are forced to join together, could produce vast amounts of energy. Most designs for fusion reactors drive the reaction by confining the fuel in a magnetic field, using a device called a tokamak.

Unfortunately, tokamaks are prohibitively heavy, so designs for fusion rockets tend to focus on another method of triggering fusion, called inertial confinement fusion.

This design replaces the tokamak's magnetic fields with high-powered energy beams, usually lasers. These blast a small pellet of fuel so intensely that the outer layers explode. This in turn crushes the inner layers, triggering fusion. Magnetic fields could then direct the resulting hot plasma out of the back of a spacecraft. Hey presto: a fusion rocket.

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